1. Field of the Invention
The present invention relates to a process of producing a glass for a soft aperture filter having a high thermal strength, and more particularly, the invention relates to a process of producing a glass for a soft aperture filter by utilizing the coloring due to silver diffused in the glass.
2. Description of the Prior Art
In general, strong focusing of light due to Fresnel diffraction at the edge of the cross section of the glass rod occurs at the center portion of the glass rod. In the case of using a laser glass, self focusing of the laser light in the laser glass occurs due to Fresnel diffraction, which results in an induction in breakage of the glass. In order to avoid such Fresnel diffraction, and prevent damage to the laser glass a soft aperture filter is used. The filter used for this purpose possesses such a function that it has a high transmission in a radius of r.sub.0 and as the radius increases, the transmission decreases gradually until the radius increases to r.sub.1 and the filter become non-transmissive of light when the radius becomes larger than r.sub.1.
The inventors have discovered previously that a soft aperture filter used for the above purpose could be produced by diffusing a coloring element such as silver, etc., in a glass as described in Japanese Patent Application No. 34613/'75 (corresponding to U.S. patent application Ser. No. 668,999 filed Mar. 22, 1976). According to an embodiment disclosed in the above patent application, a glass body 1 having a projection 1' of a form as shown in FIG. 1-(a) and FIG. 1-(b) of the accompanying drawings is immersed in fused AgNO.sub.3 contained in a vessel 2 as shown in FIG. 2 and is heat treated at a temperature near the transition point (Tg) of the glass, whereby the alkali ions in the glass are exchanged for the silver ion in the melt to cause a diffusion of the silver ion in the glass and to form a colored layer 4 of silver in the glass body 1 as shown in FIG. 3-(a). Then, by removing the projection 1' of the glass body, a glass body 1 having a colored layer portion 4, a portion 5 of which the color changes gradually, and an uncolored portion 6 as shown in FIG. 3-(b) is obtained. Finally, the colored bottom layer portion of the glass body is removed and the side surface thereof is polished to provide a soft aperture filter as shown in FIG. 3-(c).
In order to completely absorb light in the non-transmissive portion (the colored portion) of the soft aperture filter produced in the manner as described above, coloring by silver, etc., diffusion by ion exchange needs to be performed more effectively or the thickness of the colored layer portion formed by the diffusion of silver, etc. needs to be increased. As already stated in the specification of Japanese Patent Application No. 600061/'75 (corresponding to U.S. patent application Ser. No. 668,999 filed Mar. 22, 1976) to carry out more effectively coloring by silver, etc., by increasing the coloring efficiency per unit thickness of glass, a large amount of As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 must be added to the glass base. However, the thickness of the colored layer due to the silver formed in the glass body when the glass body is immersed in fused AgNO.sub.3 is inversely proportional to the acid resistance of the glass and to increase the thickness of the colored silver layer in a glass body, a glass having a low acid resistance must be used as the glass body.
Also, in forming a colored layer on the surface of a glass by immersing the glass in a silver-containing melt, a process is used wherein the glass is maintained at a temperature which is lower than the decomposition temperature of the fused salt but at a temperature at which the fused salt is maintained in a liquid state to exchange the readily soluble components (alkali ions, etc.,) in the glass for the silver ion and then the glass is heat treated again at a higher temperature to color the silver ion. However, in the production of the soft aperture filter as described above, the formation of a quite thick, colored layer of about 1 to 2 mm thick is required and for this purpose a large amount of silver must be incorporated into the glass by heat treatment of the glass at a high temperature near the transition point of the glass. However, since the decomposition temperature of AgNO.sub.3 is lower than 400.degree. to 500.degree. C., which is the transition point (Tg) of an alkali silicate glass used as the base glass for a soft aperture filter, the AgNO.sub.3 is decomposed as a result in the heat treatment at high temperature and the silver incorporation efficiency in the glass is very low.